A direct current to direct current (DC-DC) converter is a voltage converter that effectively outputs a fixed voltage after converting an input voltage, and is widely applied in products such as a mobile phone, a Moving Picture Experts Group Audio Layer III player (e.g., an MP3 player), a digital camera, and a portable media player. DC-DC converters are generally classified into a buck converter (BUCK), a boost converter (BOOST), and a buck-boost converter (BUCK-BOOST), where the BUCK is a device for converting an input DC voltage to a lower output DC voltage, and hysteretic control is a control mode of the BUCK.
FIG. 1 is a schematic structural diagram of a BUCK circuit with hysteretic control in the prior art. As shown in FIG. 1, which illustrates a BUCK circuit with hysteretic control in the prior art, a P-channel metal oxide semiconductor (PMOS) transistor 1 and an N-channel metal oxide semiconductor (NMOS) transistor 2 respectively represent a switch to a power source and a switch to the ground. When the parasitic series resistor of an inductor is not taken into consideration, the output voltage Vo and the voltage at the LX terminal are approximately equal, so that the voltage VF is a component of the output voltage Vo, that is,
            V      F        =                  V        O            *                        R          2                                      R            1                    +                      R            2                                ;when VF is lower than a reference voltage VREF of a comparator 4, the PMOS transistor 1 is turned on and the NMOS transistor 2 is turned off, and the LX terminal of the inductor L is connected to an input voltage Vin, so that the current flowing through the inductor increases, thus increasing the output voltage, and further increasing VF; when VF is increased to above VREF, the PMOS transistor 1 is turned off, and the NMOS transistor 2 is turned on, so that the current decreases, thus decreasing the output voltage, and further decreasing VF; and when VF is decreased to below VREF, the circuit repeats the foregoing process.
During the process of implementing the present invention, the inventor discovers that the prior art has at least the following technical problems: The output voltage Vo is equal to the desired output voltage
      V    REF    *                    R        1            +              R        2                    R      2      only when VF is equal to VREF, and the output voltage and the desired output voltage are unequal when VF is bigger or smaller than VREF; and in an actual circuit, the voltage value of VF is not equal to VREF because of the circuit structure and the effect of the parasitic series resistor of the inductor, so that there is an error between the output voltage and the desired output voltage.